It is actually not about trimming, pruning, wiring, etc...rather about the trunk size. Developing trunk, root/nebari, and main branches take long time. With bigger and older stock, you have that taken cared of and you just have to chop then refine. Well, I might have made it sound too simple but that is the gist if you find good materials. You may just get a stump on some (I have several of those) but there are ways to deal with it too like carving if you are into it.
You do not have to collect large trees too...there are smaller trees/bushes/shrubs with better developed trunk, nebari, & branches. I believe those are the 3 characteristics you have to concentrate on when finding a starter stock...no matter for what size. I did not mention/include leaf size just because I assume you already factored it in.
FYI, I have 8" tall plants now that started more than 10 feet. It is easy to look at the tree as a whole and buy the wrong one. CONCENTRATE on the bottom part...maybe just the 1/2 to 2/3 high of your target finished bonsai. The rest will most likely be chopped and disposed.
Good luck!